PHYLUM CHORDATA 



513 



membrane. The right and left pleural sacs are separated 

 by a considerable interval owing to the development in the 

 partition between them of a space, the mediastinum, in 

 which lie the heart and other organs. The lung is attached 

 only at its root where the pleural membrane is reflected 

 over it. In this respect it differs widely from the lung of 

 the bird. It differs also in its minute structure. The 

 bronchus entering at the root divides and subdivides to 

 form a ramifying system of tubes, each of the ultimate 

 branches of which, or terminal bronchioles, opens into a 



Fig. 308. — Lepus cuniculUS. Larynx: A. ventral view; B, dorsal view; ary, 

 arytenoid: rr, cricoid; ep, epiglottis; saitt, cartilage of Santorini; th, thyroid, 

 tr, trachea. (From Krause, after Schneider.) 



minute chamber or infundibiilum, consisting of a central 

 passage and a number of thin-walled air-vesicles or alveoli 

 given off from it. 



The spleen is an elongated, compressed, dark red body 

 situated in the abdominal cavity in close contact with the 

 stomach, to which it is bound by a fold of the peritoneum. 

 The thymus, much larger in the young rabbit than in the 

 adult, is a soft mass, resembling fat in appearance, situated 

 in the ventral division of the mediastinal space below the 

 base of the heart. The thyroid is a small brownish, bilobed 

 glandular body situated in close contact with the ventral 

 surface of the larynx. 



